The disclosure of Japanese Patent Application No. 11-329078 filed on Nov. 19, 1999, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a control apparatus and a control method for a transmission-equipped hybrid vehicle having an engine, an electric motor, and a transmission. More particularly, the invention relates to a control apparatus and a control method that improve fuel efficiency, emission characteristics, dynamics, etc.
2. Description of Related Art
A hybrid vehicle has an engine and an electric motor as drive power sources. The electric motor is preferably used not only to produce vehicle-driving torque but also as an electric power generator. In this respect, the electric motor is often termed motor-generator. The hybrid vehicle is able to improve fuel economy and the like by efficiently operating the engine and the electric motor.
A mechanical distribution type hybrid vehicle in which an engine and two electric motors are connected to a planetary gear unit has now been commercialized. This type of hybrid vehicle does not have a transmission. In contrast, a hybrid vehicle in which an engine, an electric motor, and a transmission are connected has also been proposed, as disclosed in, for example, Japanese Laid-Open Patent Application No. 8-168104. Systems equipped with continuous transmissions instead of stepwise transmissions (a type of transmission that selects one of a plurality of gear speeds) have also been proposed.
In the aforementioned mechanical distribution type hybrid apparatus, optimization in terms of fuel economy and the like is pursued by controlling the operation states of the engine and the electric motors. However, this type of hybrid apparatus does not have a transmission for selecting from gear speeds.
With regard to a vehicle that is not a hybrid vehicle, speed change characteristics of the automatic transmission are set so as to correspond to the vehicle speed and the amount of accelerator operation. Based on the speed change characteristics, a gear speed of the transmission is determined. A generally termed MMT (multimode manual transmission, that is, a transmission system where a clutch pedal is removed and clutch connecting/disconnecting operations are automatically performed by an actuator) also uses the speed change characteristics of the automatic transmission in a similar fashion to determine a gear speed.
A transmission-equipped hybrid vehicle as mentioned above can be constructed by adding to a system in which an engine and a transmission are connected as mentioned above, an electric motor that produces torque to add to the drive power of the engine. However, if the speed change characteristics of an automatic transmission intended only for use with a conventional engine are immediately applied to the transmission-equipped hybrid vehicle although the electric motor is added to the hybrid vehicle, it is impossible to select a gear speed that allows most efficient operation, and it is difficult to perform appropriate controls regarding efficiency, emissions, vehicle dynamics, etc.
Accordingly, it is one object of the invention to provide a control apparatus and a control method capable of appropriately controlling a hybrid vehicle having a transmission in terms of efficiency, emission characteristics, vehicle dynamics, etc.
To achieve the aforementioned and/or other objects, a control apparatus for a hybrid vehicle in accordance with a first aspect of the invention is for a hybrid vehicle having an engine and a motor as drive power sources and having, between the engine and a drive wheel, a transmission capable of changing drive power transmission by selection from a plurality of gear speeds. In the control apparatus, the priority sequence in a control of adjusting the drive power with respect to a vehicle drive power request is set in the sequence of an engine output increase, a motor output increase, and a gear speed change in a gear ratio increasing direction.
For example, the control apparatus of this aspect of the invention: (1) selects a gear speed of a least gear ratio within such a range that an engine revolution speed higher than or equal to a predetermined lower limit revolution speed is attainable; (2) achieves a requested drive power by an engine output with the gear speed selected; (3) achieves the requested drive power by the engine output and a motor output when the requested drive power is not achievable singly by the engine output; and (4) changes the gear speed in a gear ratio increasing direction when the requested drive power is not achievable by the engine output and the motor output.
In accordance with this aspect of the invention, the requested drive power is achieved by adjusting the engine output, the motor output, and the gear speed in that order. The gear speed change is assigned with a lower priority, and the engine output change is assigned with a higher priority. Therefore, when a drive power request is output, a control is performed so as to set the gear speed to a lower speed and increase the engine output. As a result, the engine is efficiently operated in a low-speed and high-load state, so that fuel economy improves. Furthermore, by setting the aforementioned priority sequence, the number of gear shifts is reduced. Therefore, frequent downshifts are avoided, and drivability improves, and an emission characteristic improves.
The control apparatus of this aspect of the invention may change the gear speed in accordance with a factor that affects a motor control. The factor that affects the motor control is, for example, SOC (state of charge of a battery), battery temperature, inverter temperature, etc. This aspect of the invention is able to secure a stable output supplying capability of the motor, and is able to prevent deteriorations in vehicle dynamics.
Another aspect of the invention provides a control apparatus for a hybrid vehicle having an engine and a motor as drive power sources and having, between the engine and the drive wheel, a transmission capable of changing drive power transmission by selection from a plurality of gear speeds. A gear speed of the transmission and an operation state of the engine are set such that the engine is operated in a predetermined high-efficiency operation state, and a difference between the vehicle drive power requested and an engine output is compensated by one of a drive operation of the motor and a regenerative operation of the motor. A gear speed of a least gear ratio in the transmission may be set within a range such that the engine is operated in the predetermined high-efficiency operation state, and such that the difference between the vehicle drive power requested and the engine output is compensated by one of the drive operation of the motor and the regenerative operation of the motor.
In accordance with this aspect of the invention, selecting an appropriate gear speed causes a high-efficiency operation of the engine, thereby improving fuel economy. The engine may be operated at a maximum efficiency point. Furthermore, by selecting a gear speed of a less gear ratio, improvements can be achieved in fuel economy, emissions, and drivability, as mentioned above.
The predetermined high-efficiency operation state may be a state where a multiplication product of an efficiency of the engine and a transmission efficiency of the transmission maximizes. Therefore, a gear ratio and an engine operation state are set so as to allow a maximum-efficiency operation of the engine and the transmission system as well, so that fuel economy can be improved.
Furthermore, the gear speed of the transmission and the operation state of the engine may be set such that the engine is operated in a predetermined good emission region. Therefore, an improvement can be achieved in emissions.
Still another aspect of the invention provides a control apparatus for a hybrid vehicle having an engine and a motor as vehicle drive power sources and having, between the engine and a drive wheel, a transmission capable of changing drive power transmission by selection from a plurality of gear speeds. When the drive power requested for the vehicle is negative, a gear speed of the transmission is set such that an efficiency of regenerative braking performed by the motor maximizes.
In a hybrid vehicle equipped with a transmission, the efficiency of the motor regenerative braking changes in accordance with the gear speed of the transmission. For example, when the engine is turned following the motor (i.e., when the engine is turned by the motor), selection of a less gear ratio reduces the turning resistance on the side of the engine, and therefore increases the regenerative braking efficiency on the side of the motor. Taking this fact into consideration, the apparatus of this aspect of the invention selects a gear speed such that the efficiency of regenerative braking maximizes. Therefore, improvements can be achieved in fuel economy and emissions.
The gear speed selected may vary in accordance with whether an operation of the engine is stopped or not during a regenerative operation of the motor. This aspect is intended for a construction in which the engine operation can be stopped by a clutch or the like. When the engine is turning, the turning resistance on the side of the engine decreases and the regenerative braking efficiency on the side of the motor increases with decreases in the gear ratio as stated above. When the engine is not turning, the turning resistance on the side of the engine does not need to be taken into consideration, so that a gear speed such that the efficiency on the side of the motor increases is selected. Thus, by selecting different gear speeds in accordance with whether the engine is turning or not, further improvements can be achieved in fuel economy and emissions.
The aspects of the invention are not limited to a control apparatus for a hybrid vehicle as described above. A further aspect of the invention is, for example, a hybrid vehicle or a hybrid system, and a control method for the hybrid vehicle and the hybrid system.